ktrace(1): add xref to namei(9)

namei was mistaken for a typo (see GitHub pull request #1284). Add an
xref to make it clear.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45546

(cherry picked from commit a87651e2ff189cec4c26cb220354f1bc93794f31)